96 research outputs found
On the Formation Height of the SDO/HMI Fe 6173 Doppler Signal
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the mag- netic field in
the solar photosphere. It observes the full solar disk in the Fe I absorption
line at 6173\AA . We use the output of a high-resolution 3D, time- dependent,
radiation-hydrodynamic simulation based on the CO5BOLD code to calculate
profiles F({\lambda},x,y,t) for the Fe I 6173{\AA} line. The emerging profiles
F({\lambda},x,y,t) are multiplied by a representative set of HMI filter
transmission profiles R_i({\lambda},1 \leq i \leq 6) and filtergrams
I_i(x,y,t;1 \leq i \leq 6) are constructed for six wavelengths. Doppler
velocities V_HMI(x,y,t) are determined from these filtergrams using a
simplified version of the HMI pipeline. The Doppler velocities are correlated
with the original velocities in the simulated atmosphere. The cross-
correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal
is formed rather low in the solar atmosphere. The same analysis is performed
for the SOHO/MDI Ni I line at 6768\AA . The MDI Doppler signal is formed
slightly higher at around 125 km. Taking into account the limited spatial
resolution of the instruments, the apparent formation height of both the HMI
and MDI Doppler signal increases by 40 to 50 km. We also study how
uncertainties in the HMI filter-transmission profiles affect the calculated
velocities.Comment: 15 pages, 11 Figure
GENerator of reduced Organic Aerosol mechanism (GENOA v1.0): an automatic generation tool of semi-explicit mechanisms
This paper describes the GENerator of reduced Organic Aerosol mechanism (GENOA) that produces semi-explicit mechanisms for simulating the formation and evolution of secondary organic aerosol (SOA) in air quality models.
Using a series of predefined reduction strategies and evaluation criteria, GENOA trains and reduces SOA mechanisms from near-explicit chemical mechanisms (e.g., the Master Chemical Mechanism – MCM) under representative atmospheric conditions.
As a consequence, these trained SOA mechanisms can preserve the accuracy of detailed gas-phase chemical mechanisms on SOA formation (e.g., molecular structures of crucial organic compounds, the effect of “non-ideality”, and the hydrophilic/hydrophobic partitioning of aerosols), with a size (in terms of reaction and species numbers) that is manageable for three-dimensional (3-D) aerosol modeling (e.g., regional chemical transport models).
Applied to the degradation of sesquiterpenes (as β-caryophyllene) from MCM, GENOA builds a concise SOA mechanism (2 % of the MCM size) that consists of 23 reactions and 15 species, with 6 of them being condensable. The generated SOA mechanism has been evaluated regarding its ability to reproduce SOA concentrations under the varying atmospheric conditions encountered over Europe, with an average error lower than 3 %.</p
Effects of rotational mixing on the asteroseismic properties of solar-type stars
The influence of rotational mixing on the evolution and asteroseismic
properties of solar-type stars is studied. Rotational mixing changes the global
properties of a solar-type star with a significant increase of the effective
temperature resulting in a shift of the evolutionary track to the blue part of
the HR diagram. These differences are related to changes of the chemical
composition, because rotational mixing counteracts the effects of atomic
diffusion leading to larger helium surface abundances for rotating models than
for non-rotating ones. Higher values of the large frequency separation are then
found for rotating models than for non-rotating ones at the same evolutionary
stage, because the increase of the effective temperature leads to a smaller
radius and hence to an increase of the stellar mean density. Rotational mixing
also has a considerable impact on the structure and chemical composition of the
central stellar layers by bringing fresh hydrogen fuel to the core, thereby
enhancing the main-sequence lifetime. The increase of the central hydrogen
abundance together with the change of the chemical profiles in the central
layers result in a significant increase of the values of the small frequency
separations and of the ratio of the small to large separations for models
including shellular rotation. This increase is clearly seen for models with the
same age sharing the same initial parameters except for the inclusion of
rotation as well as for models with the same global stellar parameters and in
particular the same location in the HR diagram. By computing rotating models of
solar-type stars including the effects of a dynamo that possibly occurs in the
radiative zone, we find that the efficiency of rotational mixing is strongly
reduced when the effects of magnetic fields are taken into account, in contrast
to what happens in massive stars.Comment: 11 pages, 15 figures, accepted for publication in A&
Precursors and formation of secondary organic aerosols from wildfires in the Euro-Mediterranean region
This work
aims at quantifying the relative contribution of secondary organic aerosol
(SOA) precursors emitted by wildfires to organic aerosol (OA) formation
during summer of 2007 over the Euro-Mediterranean region, where intense
wildfires occurred. A new SOA formation mechanism, H2Oaro,
including recently identified aromatic volatile organic compounds (VOCs)
emitted from wildfires, is developed based on smog chamber experiment
measurements under low- and high-NOx regimes. The aromatic
VOCs included in the mechanism are toluene, xylene, benzene, phenol, cresol,
catechol, furan, naphthalene, methylnaphthalene, syringol, guaiacol, and
structurally assigned and unassigned compounds with at least six carbon atoms
per molecule (USC>6). This mechanism
H2Oaro is an extension of the H2O
(hydrophilic–hydrophobic organic) aerosol mechanism: the oxidation of the
precursor forms surrogate species with specific thermodynamic properties
(volatility, oxidation degree and affinity to water). The SOA concentrations
over the Euro-Mediterranean region in summer of 2007 are simulated using the
chemistry transport model (CTM) Polair3D of the air-quality platform
Polyphemus, where the mechanism H2Oaro was implemented. To
estimate the relative contribution of the aromatic VOCs, intermediate
volatility, semi-volatile and low-volatility organic compounds (I/S/L-VOCs),
to wildfires OA concentrations, different estimations of the gaseous
I/S/L-VOC emissions (from primary organic aerosol – POA – using a factor of
1.5 or from non-methanic organic gas – NMOG – using a factor of 0.36) and
their ageing (one-step oxidation vs. multi-generational oxidation) are also
tested in the CTM.
Most of the particle OA concentrations are formed from
I/S/L-VOCs. On average during the summer of 2007 and over the Euro-Mediterranean
domain, they are about 10 times higher than the OA concentrations formed from
VOCs. However, locally, the OA concentrations formed from VOCs can represent
up to 30 % of the OA concentrations from biomass burning. Amongst the VOCs,
the main contributors to SOA formation are phenol, benzene and catechol (CAT; 47 %); USC>6 compounds (23 %); and toluene and xylene (12 %). Sensitivity
studies of the influence of the VOCs and the I/S/L-VOC emissions and
chemical ageing mechanisms on PM2.5 concentrations show that surface
PM2.5 concentrations are more sensitive to the parameterization used for
gaseous I/S/L-VOC emissions than for ageing.
Estimating the gaseous I/S/L-VOC emissions from POA or from NMOG has a high
impact on local surface PM2.5 concentrations (reaching −30 % in the Balkans,
−8 % to −16 % in the fire plume and +8 % to +16 % in Greece). Considering the VOC
as SOA precursors results in a moderate increase in PM2.5 concentrations
mainly in the Balkans (up to 24 %) and in the fire plume (+10 %).</p
Helioseismology of Sunspots: A Case Study of NOAA Region 9787
Various methods of helioseismology are used to study the subsurface
properties of the sunspot in NOAA Active Region 9787. This sunspot was chosen
because it is axisymmetric, shows little evolution during 20-28 January 2002,
and was observed continuously by the MDI/SOHO instrument. (...) Wave travel
times and mode frequencies are affected by the sunspot. In most cases, wave
packets that propagate through the sunspot have reduced travel times. At short
travel distances, however, the sign of the travel-time shifts appears to depend
sensitively on how the data are processed and, in particular, on filtering in
frequency-wavenumber space. We carry out two linear inversions for wave speed:
one using travel-times and phase-speed filters and the other one using mode
frequencies from ring analysis. These two inversions give subsurface wave-speed
profiles with opposite signs and different amplitudes. (...) From this study of
AR9787, we conclude that we are currently unable to provide a unified
description of the subsurface structure and dynamics of the sunspot.Comment: 28 pages, 18 figure
Complete results for five years of GNO solar neutrino observations
We report the complete GNO solar neutrino results for the measuring periods
GNO III, GNO II, and GNO I. The result for GNO III (last 15 solar runs) is
[54.3 + 9.9 - 9.3 (stat.)+- 2.3 (syst.)] SNU (1 sigma) or [54.3 + 10.2 - 9.6
(incl. syst.)] SNU (1 sigma) with errors combined. The GNO experiment is now
terminated after altogether 58 solar exposure runs that were performed between
May 20, 1998 and April 9, 2003. The combined result for GNO (I+II+III) is [62.9
+ 5.5 - 5.3 (stat.) +- 2.5 (syst.)] SNU (1 sigma) or [62.9 + 6.0 - 5.9] SNU (1
sigma) with errors combined in quadrature. Overall, gallium based solar
observations at LNGS (first in GALLEX, later in GNO) lasted from May 14, 1991
through April 9, 2003. The joint result from 123 runs in GNO and GALLEX is
[69.3 +- 5.5 (incl. syst.)] SNU (1 sigma). The distribution of the individual
run results is consistent with the hypothesis of a neutrino flux that is
constant in time. Implications from the data in particle- and astrophysics are
reiterated.Comment: 22 pages incl. 9 Figures and 8 Tables. to appear in: Physics Letters
B (accepted April 13, 2005) PACS: 26.65.+t ; 14.60.P
Cornering Solar Radiative-Zone Fluctuations with KamLAND and SNO Salt
We update the best constraints on fluctuations in the solar medium deep
within the solar Radiative Zone to include the new SNO-salt solar neutrino
measurements. We find that these new measurements are now sufficiently precise
that neutrino oscillation parameters can be inferred independently of any
assumptions about fluctuation properties. Constraints on fluctuations are also
improved, with amplitudes of 5% now excluded at the 99% confidence level for
correlation lengths in the range of several hundred km. Because they are
sensitive to correlation lengths which are so short, these solar neutrino
results are complementary to constraints coming from helioseismology.Comment: 4 pages, LaTeX file using RevTEX4, 6 figures include
Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
We study properties of waves of frequencies above the photospheric acoustic
cut-off of 5.3 mHz, around four active regions, through spatial maps
of their power estimated using data from Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory
(SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known
to capture clear oscillation signals due to helioseismic p modes as well as
waves propagating up through to the chromosphere. Here we study in detail, in
comparison with HMI Doppler data, properties of the power maps, especially the
so called 'acoustic halos' seen around active regions, as a function of wave
frequencies, inclination and strength of magnetic field (derived from the
vector field observations by HMI) and observation height. We infer possible
signatures of (magneto-)acoustic wave refraction from the observation height
dependent changes, and hence due to changing magnetic strength and geometry, in
the dependences of power maps on the photospheric magnetic quantities. We
discuss the implications for theories of p mode absorption and mode conversions
by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic
Exploration of the influence of environmental conditions on secondary organic aerosol formation and organic species properties using explicit simulations: development of the VBS-GECKO parameterization
Atmospheric chambers have been widely used to study secondary organic aerosol
(SOA) properties and formation from various precursors under different
controlled environmental conditions and to develop parameterization to
represent SOA formation in chemical transport models (CTMs). Chamber
experiments are however limited in number, performed under conditions that
differ from the atmosphere and can be subject to potential artefacts from
chamber walls. Here, the Generator for Explicit Chemistry and Kinetics of
Organics in the Atmosphere (GECKO-A) modelling tool has been used in a box
model under various environmental conditions to (i) explore the sensitivity
of SOA formation and properties to changes on physical and chemical
conditions and (ii) develop a volatility basis set (VBS)-type parameterization.
The set of parent hydrocarbons includes n-alkanes and 1-alkenes with 10, 14,
18, 22 and 26 carbon atoms, α-pinene, β-pinene and limonene,
benzene, toluene, o-xylene, m-xylene and p-xylene. Simulated SOA yields and
their dependences on the precursor structure, organic aerosol load,
temperature and NOx levels are consistent with the
literature. GECKO-A was used to explore the distribution of molar mass,
vaporization enthalpy, OH reaction rate and Henry's law coefficient of the
millions of secondary organic compounds formed during the oxidation of the
different precursors and under various conditions. From these explicit
simulations, a VBS-GECKO parameterization designed to be implemented in 3-D
air quality models has been tuned to represent SOA formation from the 18
precursors using GECKO-A as a reference. In evaluating the ability of
VBS-GECKO to capture the temporal evolution of SOA mass, the mean relative
error is less than 20 % compared to GECKO-A. The optimization procedure
has been automated to facilitate the update of the VBS-GECKO on the basis of
the future GECKO-A versions, its extension to other precursors and/or its
modification to carry additional information.</p
Surprising Sun
Important revisions of the solar model ingredients appear after 35 years of
intense work which have led to an excellent agreement between solar models and
solar neutrino detections. We first show that the updated CNO composition
suppresses the anomalous position of the Sun in the known galactic enrichment.
The following law: He/H= 0.075 + 44.6 O/H in fraction number is now compatible
with all the indicators. We then examine the existing discrepancies between the
standard model and solar - seismic and neutrino - observations and suggest some
directions of investigation to solve them. We update our predicted neutrino
fluxes using the recent composition, new nuclear reaction rates and seismic
models as the most representative of the central plasma properties.
We get 5.31 +- 0.6 10^6/cm^{2}/s for the total ^8B neutrinos, 66.5 SNU and
2.76 SNU for the gallium and chlorine detectors, all in remarquable agreement
with the detected values including neutrino oscillations for the last two. We
conclude that the acoustic modes and detected neutrinos see the same Sun, but
that clear discrepancies in solar modelling encourage further observational and
theoretical efforts.Comment: 4 pages 3 figures Submitted to Phys. Rev. let
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